1. Field of the Invention
The present invention relates to a photoelectric converter made of a semiconductor transistor of the type wherein the potential of a control electronic region is controlled through a capacitor.
2. Related Background Art
FIG. 5A is a schematic plan view showing the pattern of a conventional photoelectric converter, and FIG. 5B is a sectional view showing a photoelectric converter cell as taken along line A--A' of FIG. 5A.
In FIGS. 5A and 5B, formed on an n-type silicon substrate 1 is an n.sup.- epitaxial layer 4 within which photoelectric conversion cells are formed to be electrically insulated from each other by element isolation regions 6.
Formed on the n.sup.- epitaxial layer 4 is a p.sup.- base region 9 within which an n.sup.- emitter region 15 is formed. A capacitor electrode 14 is formed on the p.sup.- base region 9 with an oxide film 12 interposed therebetween to control the potential of the p.sup.- base region 9. An emitter electrode 19 is formed to be connected to the n.sup.+ emitter region 15.
Also formed are an electrode connected to the capacitor electrode 14, an n.sup.+ region 2 at the back of the substrate 1 for use in ohmic contact, and a collector electrode 21, respectively for the bipolar transistor. Thus, a photoelectric conversion cell is constructed.
In fundamental operation of a photoelectric conversion cell, first the p.sup.- base region 9 biased negative is made float so that the holes of light-induced electron/hole pairs are stored in the p.sup.- base region 9 (storage operation).
Succeedingly, a positive pulse is applied to the capacitor electrode to forward bias the emitter-base junction and read a voltage generated by the stored holes from the floating emitter (readout operation).
Next, the emitter is grounded and the capacitor electrode 14 is supplied with a positive pulse to thereby remove the stored holes in the p.sup.- base region 9 When the positive refresh pulse falls, the p.sup.- base region 9 recovers its initial state (refresh operation).
In such a photoelectric converter, stored charges are amplified using the amplification function of each cell and thereafter they are read. Therefore, a high output power, high sensitivity and low noise can be achieved. Moreover, since it has simple construction, it is advantageous for high resolution of photoelectric converters in future. However, the conventional photoelectric converter has such a construction that the capacitor electrode 14 faces the base region 9 of low impurity density. Thus, there arises a problem that the operation is not stable to thereby cause read-out signals to fluctuate.
Namely, depending upon a voltage pulse applied to the capacitor electrode 14, the interface between the oxide film 12 and the p.sup.- base region 9 is subjected to storage, depletion and inversion state one after another. As the state changes, the capacitance changes which may cause a dark current.
Further, in case a plurality of photoelectric conversion cells are arrayed, all the cells are not subjected to a uniform change of the state. Therefore, read-out signals are caused to fluctuate and a fixed pattern noise may be generated.